diff --git a/src/PIL/GifImagePlugin.py b/src/PIL/GifImagePlugin.py
index bdea02005e7..0793d4b4277 100644
--- a/src/PIL/GifImagePlugin.py
+++ b/src/PIL/GifImagePlugin.py
@@ -656,7 +656,7 @@ def _save(im, fp, filename, save_all=False):
         palette = im.encoderinfo.get("palette", im.info.get("palette"))
     else:
         palette = None
-        im.encoderinfo["optimize"] = im.encoderinfo.get("optimize", True)
+        im.encoderinfo.setdefault("optimize", True)
 
     if not save_all or not _write_multiple_frames(im, fp, palette):
         _write_single_frame(im, fp, palette)